Of the rotary encoders used to control industrial equipments such as numerically controlled machine tools and machine tool robots, optical encoders which are susceptible to electrical noise have gained wide acceptance.
The structure of such an optical rotary encoder is shown in FIGS. 1 and 2, where the rotor 1 of the encoder is connected to the shaft of a rotary driving device (not shown). The rotor 1 has a plurality of circumferentially spaced reflectors 3 on the outer periphery of the body 2 of the rotor 1. Usually, the reflectors 3 are equally spaced apart from one another. A sensor head 4 has a light projector 5 and a light receiver 6 both of which are disposed opposite to the reflectors 3 of the rotor 1. An optical-fiber cable 7 for transmitting light terminates in the light projector 5. Another optical fiber cable 8 for receiving light terminates in the light receiver 6.
In the operation of the conventional optical rotary encoder constructed as described above, when the rotary driving device is set in motion, the rotor 1 is rotated by the shaft of the driving device. Then, the light which is projected from the light projector 5 through the fiber 7 on the area across which the reflectors 3 pass is intermittently reflected by the reflectors 3 and encoded. The encoded signal is received by the light receiver 6 and guided to the fiber cable 8. Then, it is taken from the cable as a control signal.
The performance of this conventional encoder depends strongly on the state of the reflectors 3, and therefore it is necessary to prevent corrosion of the reflectors 3 during longtime use for avoiding decrease in the reflectivity. The performance of the encoder also depends on the accuracy of intervals at which the reflectors 3 are disposed. Hence, it is also necessary to enhance this accuracy.